7.09 It is interference by a batter or a runner when-. . .(h) In the judgment of the umpire, the base coach at third base, or first base, by touching or holding the runner, physically assists him in returning to or leaving third base or first base.. . .PENALTY FOR INTERFERENCE: The runner is out and the ball is dead.

If a coach taps a players to communicate, is it interference? I've always ruled this as legal, but that seems to be a minority opinion. I think this rule only applies when the coach accelerates the player's body. Other people rule any touch that assists the runner, even if that assistance is information,

If everybody is thinking the same thing, then somebody isn't thinking.- General George Patton

I think it matters if a play is actually still going on in the field. If it is between pitches, or a play is dead, it should be allowed to tap a player to communicate with them.The rule you stated is that the contact has to assist a player, returning or leaving a base.